Amplifier circuit for testing



Q. A. KERNS AMPLIFIER CIRCUIT FOR TESTING Original Filed June 19. w47

AAAAAA Feb. 10, 1953 IN V EN TOR. QUENr//v A KERNS ATTORNEYl Patentecl Feb. 10, 1953 AMPLIFIER CIRCUIT FOR TESTING Quentin A. Kerns, Oakland, Calif., assigner to the United States of America as represented by the United States Atomic Energy Commission Original application June 19, 1947, Serial No. 755,794. Divided and this application October 4, 1949, Serial N0. 119,539

3 Claims. 1

This invention relates to an amplier circuit and more particularly to a negative feedback amplifier circuit for use in determining the current-voltage characteristics of electrical devices. This application is a division of the applicants copending application, Serial No. 755,794, filed June 19, 1947.

In the electrical art it is often desirable to know the current-voltage characteristics of electrical devices such as electrolytic cells, discharge devices and the like. The present invention obviates the necessity of tedious manual work in obtaining current readings at various volta-ges and then plotting the results on a chart, by providing a circuit for automatically displaying or recording the current-voltage characteristics. The circuit herein d-isclosed further provides for minimum distortion in the characteristic by utilizing a novel negative feedback amplier circuit having a novel input circuit.

It is therefore an object of the present invention to provide a new .and improved amplider circuit.

Another object of the invention is to provide a new and improved circuit for automatically determining the current-voltage characteristics of an electrical device.

A further object of the invention is to provide an input circuit for an amplifier having a negative feedback circuit whereby the input is distorted a minimum amount at the output.

Still another object of the invention is to provide a negative feedback .ampliiier circuit wherein the current-voltage characteristics of an electrical device connected into the negative feedback circuit is automatically determined.

Still further objects and advantages of the invention will be'apparent in the following description and claims considered together with the accompanying drawing showing a schematic wiring diagram embodying the invention.

Referring to the drawing in detail there are shown three pentode vacuum tubes III, II2 and H3, which give three stages of amplication, and a -pentode tube I I 4 which serves to couple the input voltage to the amplifier stages. An input lead IIB is connected to the control grid of the tube I It and supplies a voltage of sawtooth waveform from a voltage generator (not shown). Other connections of the tube I I4 are: from the anode to a +280 volt source of direct current through a dropping resistor I I'I and to ground through a lIcy-pass condenser I I8 ;l from the screen E 40 2 grid to the +280 volt source of direct current Athrough la. dropping resistor I I9 and to ground through a by-pass condenser IZB; and from the suppressor grid to the cathode, the latter of which is connected to a -150 volt source of direct current through a cathode resistor |22.

The cathode of the tube III is connected to the suppressor grid and to the cathode of the tube H4 through a resistor I2I. A lconnection is made from the screen grid of the tube I II to the screen grid of the tube II4. The anode of the tube I I I is connected to the +280 volt direct current source through a dropping resistor |23 and, further, to the control grid of the tube II2 through a low impedance coupling network |24 comprising a parallel connected condenser I 25 and resistor |26. For furnishing a variable bias voltage to the control grid of the tube II2, there is provided a resistor I2'I -connected to the variable portion of a resistance voltage divider comprising a potentiometer |28 and a resistor I 23 between the -150 volt direct current source and ground. Other connections provided for the tube H2 are: from the cathode to the suppressor grid and to ground through a resistor |34, from the screen grid directly to a +150 volt direct current source, and from the anode to the +280 volt direct ycurrent source through a, dropping resistor I35.. A further connection is made from the anode of the tube II2 to the control grid of the tube II3 through a low impedance coupling network I36 comprising a condenser I3`I and a resistor I 38. For furnishing the necessary bias voltage for the control grid of the tube I I3, a resistor |39 is connected between the control grid and the 150 volt direct current source. The suppressor grid and cathode of the tube I I3 are connected together and inturn connected directly to ground. The screen grid of the tube IIS is supplied with Ia lsuitable voltage by a direct connection to the +150 volt direct current source. A suitable anode voltage for the tube II3 is sup plied from the +280 volt direct current. source through a dropping resistor |49. The anode of this tube II3 serves as thefsource of output voltage which is removed by a lead IM through a lcoupling network 14| comprising a parallel connected condenser |42 and neon bulb M3. The lead IM is further connected to a 200 volt source -of direct current .through v-a resistor M5. A portion of the output voltage at the lead IM is fed back to the cathode ofthe tube I I2 through a coupling network |46 comprising a parallel lconnected condenser |4'| land resistor |48. Also provided as a feedback circuit is a connection from the lead |44 to the control grid of the tube through a Vcoupling network |49 comprising the condenser |5| and one of a plurality of resistors |52 to |54, |56 to |59, |5| Vand |52 as selected by a selector switch |63. One terminal of the selector switch is connected directly to ground, for use in Ishort-circuiting the feedback Voltage when desired. As a means of connecting a device |54, of which the current-voltage characteristic is desired, across the feedback voltage of the amplien there is provided a plug |65.

Now, consider the operation of the present invention with the necessary voltage sources energized and an electrical device |64, the currentvoltage characteristic of which is desired, connected between the plug |65 and ground. Under these conditions, the circuit is ready to operate, and a positive-going sawtooth wave of voltage impressed by the lead ||6 on the control grid of the tube ||4 will impress a similar voltage at the cathode of the tube This action will be apparent in considering the interconnection of the cathodes of the two tubes I4 and which have a common cathode resistor |22. To insure that the input voltage is free from distortion the circuit has been designed to include a common screen grid supply voltage which is by-passed to ground by a condenser |20. Also, the two tubes and ||4 are chosen to be of a similar type and a further measure is taken by br-passing variations in the anode voltage of the tube ils with a. condenser HB. Thus, the input circuit is efiectively isolated from distortion from the various power sources.

The positive-going sawtooth of voltage thereby raises the cathode voltage of the tube iii, reduces the conduction of the tube and causes a decreasing voltage across the dropping resistor |23 in the anode circuit of the tube. Thus, the

scitage at the anode of the tube l increases and a positive-going sawtooth voltage, similar to that t the cathode of the tube, is formed.

Since the anode of the tube is connected to the control grid of the tube l2 by the condenser |25 and resistor |25 of network |213, the positivegoing sawtooth voltage developed at the anode of the tube is coupied to the control grid of the tube HE to control the operation thereof. The control grid of the tube H2 is also connected to a variable voltage so that the grid bias voltage is controllable. The tubes i2 and |3 are interconnected in such a manner as to form two stages or amplification of the positive-going sawtooth voltage formed at the control grid of the tube 2.

The output voltage of the tube llt is taken from the anode and a feedback circuit arranged to couple portions of this output voltage back to the cathode of the tube l2 and to the control :grid of the tube lil. The portion of the output voltage impressed at the control grid o the tube l is coupled thereto through a resistor, conand switch arrangement ifi-9 in such a manner that the voltage is controllable in a desired incremental manner as is the over-all pliiication of the circuit.

From the foregoing, it is seen that a positivegoing sawtooth voltage is impressed across the device i6@ which is connected between the control grid of the tube and ground and that the voltage applied to the amplifier is therefore proportional to the current flowing through the device. The output voltage at the lead |44 will then 4 be proportional to the sum of the input voltage and the voltage across the device |64.

It is to be noted that the operating voltages in the foregoing discussion, referenced by numerical values, are used only for illustrative purposes and should not be considered as limiting the values of these voltages in any manner.

An illustrative set of 'values for he different circuit elements of the invention is listed below, but should not be considered as limiting the invention to such values.

||| Tube, 6SH7 |2 Tube, 65H7 ||3 Tube, GSH? Resistor, 30,000 ohm, 2 w. ||8 Condenser, .005 mfd., 600 v. I9 Resistor, 50,000 ohm, l w. |20 Condenser, .005 mfd., 600 v. |2| Resistor, 5 ohm, 1/2 w.

|22 Resistor, 12,000 ohin, 2 w. |23 Resistor, 30,000 ohm, 2 w. |25 Condenser, 10 mmfd.

|26 Resistor, 3 megohm, l/; w. |21 Resistor, 2 megohrn, 1/,1 it'. |23 Resistor', 0.1 inegohm, l tv. |29 Resistor, 0.2 inegohm, 1/2 w. |34 Resistor, 500 ohm, l 's'.

|35 Resistor, 10,000 ohm, l/ w. |31 Condenser, 10 minfd.

|38 Resistor, .i megohm, s'.

i |39 Resistor, 2 niegohm, 1,/2 w. |40 Resistor, 30,000 ohm, 2 w. M2 Condenser, .0l mid., 500 v. |43 Neon lamp, IUE-4. |45 Resistor, 0.5 megolim, t@ u'. |5| Condens r, l0 mind.

|52 Resistor, 40,000 ohm,

153 Resistor, 80,005 ohm, w. ld Resistor, 0.2 megohin, vf. |56 Resistor, 0.4 niegohin, ,/2 |51I Resistor, 0.8 megohm, lf2 w, Resistor', 2.0 inego'tzzi, 1/2 w. |53 Resistor, 4.0 inegohin, 1/'3 vf. |5| Resistor, 8.0 inegolnn, 1,1?, w. |62 Resistor, 20.0 megohm, V2 vx.

One manner of using; the present invention is described and illustrated in the above-referenced copending application, Serial No. 755,794, led June 19, 1047, by the applicant. In this utilization, the current-voltage characteristics of an electrolyte in a dropping mercury cell is determined.

A further utilization for the present invention is the determination of the current-voltage characteristics of electron discharge devices of the vacuum tube class. In this instance, the anode is connected to the plug |55, the cathode is connected to ground and the control grid is connected to a suitable bias voltage. Thus, a positive-going sawtooth voltage impressed at the control grid of the tube I4, will apply a similar voltage between the anode and cathode of the electron discharge device. At a constant grid bias voltage, the current iiowing through the tube will vary according to the positive-going sawtooth voltage applied. Therefore, the voltage which is impressed at the control grid of the tube is proportional to the current fiowing through the device and is available at the output lead |44 of the amplifier.

There are many ways of measuring or recording the current-voltage characteristic of the electron discharge device, now that a voltage has been developed which is proportional tothe current flowing, and one such way is to impress the output voltage of the ampliiier on one set of deecting plates of an oscilloscope with the sawtooth voltage impressed on the other set of deiiecting plates. in this manner the current-voltage characteristics are obtained directly and may be photographed for a permanent record.

While the present invention has been described with respect to one embodiment it will, of course, f

be apparent that many modifications may be made within the spirit and scope of this invention and it is therefore not desired to limit the invention to the exact details except insofar as they may be dened in the following claims.

What is claimed is:

1. In apparatus for measuring current-voltage characteristics, the combination comprising an ampliiier having an input and an output, a first and second vacuum tube, each of said tubes having at least a cathode, a control grid, and an anode, two equal resistances for connecting the anodes of the respective tubes to a positive voltage, a common resistance connecting said cathodes to a negative voltage, the anode of said second tube being connected to the input of said amplifier, a negative feedback circuit for connecting the output of said amplier to the control grid of said second tube, and a device having characteristics which it is desired to determine connected between the control grid of said second tube and a neutral voltage, whereby a voltage proportional to the current through said device appears at the output of said amplifier each time a linearly varying voltage is applied to the control grid of said rst tube. y

2. In a measuring apparatus, the combination comprising a rst vacuum tube, a source of input voltage connected to the control grid of said rst tube, a second vacuum tube, said rst and second tubes having a common cathode resistor, a third Vacuum tube, the control grid of said third tube being coupled to the anode of said second tube, a fourth vacuum tube, the control grid of said fourth tube being coupled to the anodev of said third tube, a variable resistance-capacitance network coupled to the anode of said fourth tube and connected to the control grid of said second tube, an electrical device having unknown characteristics connected to the control grid of said second tube, and power supply means connected to said tubes to bias said first, second, and fourth tubes nonconducting and said third tube conducting whereby a voltage proportional to the current flow through said device is developed at the anode of said fourth tube each time a linearly varying voltage is applied to said input lead.

3. In a measuring apparatus, the combination comprising a first and second vacuum tube, each of said tubes having at least a cathode, a control grid, and an anode, a common resistance connected to the cathode of said tubes, means connected to said tubes for impressing operating voltages thereon, a device having characteristics which it is desired to determine connected to the control grid of the second of said tubes, an amplifier having an input and an output, the anode of the second of said tubes being connected to the input of said amplier, and adjustable means connected between the output of said amplifier and the control grid of said second tube for impressing a portion of the output of said amplifier across said device whereby a voltage proportional to the current flow through said device appears at the output of said amplier each time a linearly varying voltage is impressed on the control grid of said iirst tube.

QUENTIN A. KERNS.

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 2,185,367 Blumlein Jan. 2, 1940 2190,743 Vance Feb. 20, 1940 2,401,779 Swartzel June 11, 1946 2,431,973 White Dec. 2, 1947 2,470,219 McNaughton May 17, 1949 2,475,188 Krauth July 5, 1949 2,479,909 Darlington Aug. 23, 1949 

